Speed control system for alternating current induction motors

ABSTRACT

A speed control system for alternating current induction motors wherein a reference potential, which is a rectified phase of the alternating current supply potential, is connected across the series combination of a variable resistor and the timing capacitor of a unijunction transistor type relaxation oscillator circuit and a direct current control potential, which is a function of motor speed and which may be selectively varied in magnitude to change the speed of the motor, is connected across the variable resistor in the same polarity relationship as the reference potential. The unijunction transistor type relaxation oscillator circuit and associated circuitry produces a trigger signal at the electrical angle of each potential cycle of each phase of the alternating current supply potential as determined by the difference of magnitude between the reference and control potentials which trigger respective silicon controlled rectifiers conductive to complete an energizing circuit for the corresponding phase winding or the motor. Another capacitor, connected across the series combination of the variable resistor and the timing capacitor, establishes the difference of magnitude between the reference and control potentials at substantially zero upon motor start and increases the difference at a predetermined rate to a maximum as determined by the selected magnitude of the control potential.

United States Patent Pryjmak Y [54] SPEED CONTROL SYSTEM FOR I.

ALTERNATING CURRENT INDUCTION Primary Examiner-Gene Z. RubinsonAttorney-Eugene W. Christen, Creighton R. Meland and Richard G. StahrABSTRACT A speed control system for alternating current induction motorswherein a reference potential, which is a 3,700,988 {431 Oct. 24,1972

rectified phase of the alternating current supply potential, isconnected across the series combination of a variable resistor and thetiming capacitor of a unijunction transistor type relaxation oscillatorcircuit and a direct current control potential, which is a function ofmotor speed and which may be selectively varied in magnitude to changethe speed of the motor, is connected across the variable resistor in thesame polarity relationship as the reference potential. The unijunctiontransistor type relaxation oscillator circuit and associated circuitryproduces a trigger signal at the electrical angle of each potentialcycle of each phase of the alternating current supply potential asdetermined by the difference of magnitude between the reference andcontrol potentials which trigger respective silicon controlledrectifiers conductive to complete an energizing circuit for thecorresponding phase winding or the motor. Another capacitor, connectedacross the series combination of the variable resistor and the timingcapacitor, establishes the difference of magnitude between the referenceand control potentials at substantially zero upon motor start andincreases the difference at a predetermined rate to a maximum asdetermined by the selected magnitude of the control potential.

3 Claims,-3 Drawing Figures TO ONE PHASE OF SUPPLY POTENTIAL MONOSTABLEMULTIVIBRATOR MONOSTABLE MULTIVIBRATOR 76 L a Jqm- 5 P'AIEMTEDUm 24 I972SHEET 1 OF 2 TO ONE PHASE OF SUPPLY POTENTIAL MONOSTA Bl. E

MULTIVIBRATOR MONOSTABLE MULTIVIBRATOR I NVENTOR. 50/2022! Z A WJM/ATTORNEY energized by the phase of the alternating current supplypotential to which they'correspond. The number of coils per phasewinding may vary depending upon the number of poles designed into themotor. Motors of this type normally operate at a constant, fixed speedwhich is determined by the frequency of the alternating current supplypotential and the number of magnetic poles'produced by the phasewindings.

In certain applications where motors of this type may be advantageouslyused, it may be desirable to operate the motor at variable speeds. Tochange the speed of al- I 'ternating current induction motors, it hasheretofore been necessary to change the frequency of the alternatingcurrent supply potential or the number of magnetic poles produced by thephase windings. Both of these alternatives have been unsatisfactory inthat that the former requires expensive frequency converting equipmentand the latter provides, at best, step-by-step control through compoundswitching arrangements.

It has been found that the speed of alternating cur-' rent inductionmotors may be smoothly adjusted over a wide range by controlling theelectrical conduction angle during which each potential cycle of eachphase of the alternating current supply potential energizes thecorresponding phase windings. To reduce the speed, the respective phasewindings are energized over a shorter electrical conduction angle ofeach potential halfcycle of the corresponding phase. For optimumoperation at any speed, the conduction angle of the potential cycles ofall phases should be equal to the conduction angle of the potentialcycles of one of the phases selected to be the reference phase. I

In another United States patent application, Ser. No.

493,652, filed Oct. 7, 1965, now US. Pat. No.

3,387,196, and assignedto the same assignee as that of the presentapplication, a novel speed control system which may be used with anyalternating current induction motor of the type having discrete phasewindings corresponding to each phase of a compatible alternating currentsupply potential is disclosed in detail. Briefly, the speed controlsystem so disclosed is responsive to a difierence of magnitude betweenadirect current reference potential and a direct current controlpotential, which is not only a function of motor speed but is alsoindependently variable to vary the speed of the motor, to produce anelectrical trigger signal at the electrical angle of each potentialhalf-cycle of each phase of the alternating current supply potential asdetermined by the selected motor speed for operating respective triggersignal responsive switching devices which establish respective phasewinding energizing circuits when operated to close by the triggersignals. In this system, the greater the difference of magnitude betweenthe reference and control potentials, the earlier during each potentialhalf-cycle of each phase of the alternating current supply potential thecorresponding :motor winding is energized to increase the speed ofthe'motor. 'Upon motor start, therefore, with a control potential. ofzero, e'ach motorphase winding is energizedsubstantially at thebeginning of each potential cycle of the corresponding phase of thesupply potential. Consequently, substantially full supply potential isapplied acrossthe phasewindings of the motor at start. In manyinstances, particularly with integral horsepower motor ratings, this maycause intolerable transient conditions on the supply line.

It is, therefore, an object of this invention to provide an improvedmotor speed control system for altemating current induction motors ofthe type which produces electrical trigger signals at the electricalangle I of each potential cycle of each phase of the alternating currentsupply potential as determined by the difference of magnitude between adirect current reference potential and a direct current controlpotential which are applied across the gate-cathode electrodes ofrespective silicon controlled rectifiers for triggering these devicesconductive to complete an ener- 'gizing circuit for the correspondingphase winding.

It is another object of this invention to provide an improved speedcontrol system for alternating current induction motors of the typewhich produces an electrical trigger signal at the electrical angle ofeach potential cycle of 'each phase of the alternating current supplypotential as determined by the difference of magnitude between a directcurrent reference potential and a direct current control potential fortriggering respective silicon controlled rectifiers conductive tocomplete an energizing circuit for the respective phase windings of themotor wherein the difference of magnitude between the reference andcontrol potentials is established at substantially zero upon motor startand is increased at a predetermined rate to a maximum as determined bythe magnitude of the control potential.

In accordance with this invention, a motor speed control system foralternating current induction motors of the type which produces anelectrical trigger signal at the electrical angle of each potentialcycle of each phase of the altemating' current supply potential asdetermined by the difference of magnitude between a direct currentreference and a direct current control potential for triggeringrespective silicon controlled rectifiers conductive to complete anenergizing circuit for the corresponding phase winding of the motor isprovided wherein the difference of magnitude between the reference andcontrol potentials is established at substantially zero upon motor startand is increased at a predetermined rate to a maximum as determined bythe selected magnitude of the control potential.

For a better understanding of the present invention, together withadditional objects, advantages and features thereof,- reference is madeto the following description and accompanying drawings in which:

FIG. 1 is a schematic diagram of the improved motor speed control systemfor alternating current induction motors of this invention;

FIG. 2 is a schematic diagram of the switching system for a wyeconnected alternating current induction motor which may be used with thecircuitry schematically set forth in FIG. 1; and

FIG. 3 is a schematic diagram of the switching system for a deltaconnected alternating current induction motor which may be used with thecircuitry schematically set forth in FIG. 1.

The novel speed control system of this invention may I be employed withany alternating current induction motor of the type having discretephase windings corresponding to each phase of a compatible alternatingto phase A and, when conducting, completes an energizing circuit for thephase windings corresponding to a direct current reference potential anda direct current control potential for producing a trigger signal at theelectrical angle of each potential cycle of each phase of thealternating current supply potential as determined by the magnitude ofthe difierence.

The circuitry represented by blocks 34 and 35 may be conventionalmonostable multivibrator circuits well phase A through the phasewindings corresponding to phase B. Silicon controlled rectifier llcorresponds to phase B and, when conducting, completes an energizingcircuit for the phase windings corresponding to phase B through thephase windings corresponding to phase C. Silicon controlled rectifier 12corresponds to phase C and, when conducting, completes an energizingcircuit for the phase windings corresponding to phase C through thephase windings corresponding to phase A. With both silicon controlledrectifiers l0 and ll conducting, phase A may feed both phases B and C;

with both silicon controlled rectifiers 11 and 12 conducting, phase Bmay feed both phases C and A and with both silicon controlled rectifiers12 and 10 conducting, phase C may feed both phases A and B.

In FIG. 3, the parallel combination of oppositely poled siliconcontrolled rectifiers 13 and 14 correspond to phase A and are connectedin series with the phase controlled rectifiers l7 and 18 correspond tophase C j and are connected in series with the responding to phase C.

With single and two-phase applications, one and two switching devices,respectively, are required. With applications of more than three phases,one or two switching devices corresponding to each phase may be requireddepending upon the configuratiori in which the phase windings areconnected.

As the point of reference or ground potential is the same pointelectrically throughout the system, it has been illustrated by theaccepted schematic symbol and referenced by the numeral 5 in FIG. 1 ofthe drawings.

That portion of the circuit of FIG. 1 including a unijunction transistortype relaxation oscillator circuit comprising unijunction transistor 20,timing capacitor 25 and base resistors 22 and 23; type NPN transistor 30and transformer 26 having a primary winding 27 and two secondarywindings 28 and 29 and the blocks referenced by the numerals 34 and 35comprise circuitry responsive to a difference of magnitude between phasewinding corknown in the art. Consequently, as these circuits per se formno part of this invention, each has been illustrated in block form.

The circuitry comprising the unijunction transistor type relaxationoscillator circuit, type PNP transistor 30 and transformer'26 is amaster trigger signal producing circuit which produces a trigger signalat the electrical angle of each potential cycle of the reference phaseof the supply potential as determined by the magnitude of the differencebetween the reference and control potentials.

Each of monostable multivibrator circuits 34 and 35 is a slave triggersignal producing circuit connected in cascade with the master triggersignal producing circuit and responsive to the trigger signal producedby the next preceding trigger signal producing circuit to produce atrigger signal corresponding to a respective other-phase of thealternating current supply potential and spaced from each other by anelectrical angle equal to the electrical angle between phases of thealternating current supply potential.

To produce a direct current reference potential in synchronism with aselected phase of the supply potential, one phase of the alternatingcurrent supply potential may be rectified bya full-wave diode bridgerectifier circuit, generally shown by reference numeral 8 in FIG. 1 andhereinafter referred to as the source of reference potential, whichproduces a pulsating direct current potential, hereinafter referred toas the reference potential. 1

The reference potential, which may be regulated by a Zerier diode 24, isconnected across the series combination of an impedance element, whichmay be a variable resistance 36, and timing capacitor 25 through acircuit which may-be traced from the positive polarity direct currentoutput terminal of rectifier circuit 8, through resistor 37, lead 38,the parallel combination of capacitor 44, resistor 45 anddiode 46, anddiode 47, potentiometer 48 and resistor 49 in series, through van'ableresistance 36, lead 50, timing capacitor 25 and point of reference orground potential 5 to the negative polarity direct current outputterminal of rectifier circuit 8.

Timing capacitor 25 is charged by the reference potential and, when thecharge thereupon which is applied across the emitter electrode 21 andone of the base electrodes of unijunction transistor 20, reaches amagnitude substantially equal to the peak point potential of unijunctiontransistor20, this device switches to a low resistance or conductingstate. With unijunction transistor 20 conducting, timing capacitor 25discharges through the emitter-base junction of unijunction transistor20 and base resistor 23. When the potential applied to emitter electrode21 decreases to a value which is of insufficient magnitude to maintainunijunction transistor 20 conducting as timing capacitor 25 discharges,the unijunction transistor 20 reverts to its high resistance ornonconducting state and the cycle just described is repeated. Upon eachwhich is of a positive polarity upon junction 51 with respect to pointof reference or ground potential 5.

complete an energizing circuit for phase winding 8 through phase windingC. The potential'induced in secondary winding 69 triggers monostablemultivibra- As onlyone trigger signal is required during each potentialcycle of each phase and since the relaxation oscillator circuit isnormally free-running, as described in the preceding paragraph, it isnecessary that the relaxation oscillator circuit be disenabled duringthe remainder of each potential cycle after a trigger signal hasbeen'produced. To disenable the relaxation oscillator circuit upon theproduction of a trigger signal, silicon controlled rectifier 40 isprovided. The trigger signal appearing across resistor 23 is appliedthrough coupling capacitor 52 across the gate-cathode electrodes ofsilicon controlled rectifier 40 in the proper polarity relationship toproduce conduction through the anode-cathode electrodes thereof.Conducting sil- 'icon controlled rectifier 40 removes the interbasepotential from unijunction transistor 20 as conducting siliconcontrolled rectifier 40 is substantially a short circuit across thedirect current terminals of rectifier bridge 8.

The trigger signal appearing across base resistor 23 is also appliedthrough coupling capacitor 52 across the base electrode 31 and emitterelectrode 33 of type NPN transistor 30 in the proper polarityrelationship to produce baseemitter current flow through this type ofNPN transistor. The resulting current flow through collector electrode32 and emitter electrode 33 completes an energizing circuit for primarywinding 27 of transformer 26. As the flow of current through primarywinding 27 increases frornzero, a potential is induced in each ofsecondary windings 28 and 29. The potential induced in secondary winding28, which is poled in such a manner that the potential upon outputterminal 53( 1) is of a positive polarity with respect to outputterminal 54(1), is applied across the gate-cathode electrodes of siliconcontrolled rectifier of FIG. 2 through output terminals 53(1) and 54(1)of FIG. 1 and input terminals 53(2) and 54(2) of FIG. 2, respectively,in the proper polarity relationship to trigger silicon controlledrectifier l0 conductive to complete an energizing circuit for phasewinding A through phase winding B. The potential induced in secondarywinding 29 triggers monostable multivibrator 34 to the alternate stateto complete an energizing circuit for primary winding 67 of transformer66. Monostable multivibrator 34 remains in the alternate state for aperiod of time corresponding to 120 electrical degrees, the number ofelectricaldegrees between phases of a threephase supply potential, andspontaneously returns to the stable state to interrupt the energizingcircuit for primary winding 67. The resulting collapsing magnetic fieldinduces a potential in each of secondary windings 68 and 69,. Thepotential induced in secondary winding 68, which is poled in such amanner that the potential upon output terminal 55(1) is of a positivepolarity with respect to output terminal 56(1), is applied across thegate-cathode electrodes of silicon controlled rectifier 11 of FIG. 2through output terminals 55(1) and 56(1) of FIG. 1 and input terminals55(2) and 56(2) of FIG. 2, respectively, in the proper polarityrelationship to trigger silicon controlled rectifier 11 conductive totor 35 to the alternate state to complete an energizing circuit forprimary winding 77 of transformer 76. Monostable multivibrator 35remains in the alternate state for a period of time corresponding to 120electrical degrees and spontaneously returns to the stable state tointerrupt the energizing circuit for primary winding 77. The resultingcollapsing magnetic field induces a potentialin secondary winding 78 ofa positive polarity upon output terminal 57(1) with respect to outputterminal 58(1) which is applied across the gatecathode electrodes ofsilicon controlled rectifier 12 of FIG. 2 through output terminals 57(1)and 58(1) of FIG. 1 and input terminals 57(2) and 58(2) of FIG. 2,respectively, to trigger this device conductive to complete anenergizing circuit for phase winding C through phase winding A. Thecircuitry of FIG. 1 may also be used with a delta connected inductionmotor as shown in FIG. 3. To connect the configuration of FIG. 3 intothe circuitry of FIG. 1, coils 59 and 60 of FIG. 3 are magneticallycoupled to primary winding 27 of FIG. 1 in place of secondary winding28, coils 61 and 62 of FIG. 3 are magnetically coupled to primarywinding 67 of FIG. 1 in place of secondary winding 68 andcoils 63 and 64of FIG. 3 are magnetically coupled to primary winding 77 of FIG. 1 inplace of secondary winding 78. The coils of FIG. 3 are, of course, sopoled to produce the correct potential polarity relationship across thegate-cathode electrodes of each respective silicon controlled rectifier.A complete detailed description of this circuit is set forth in theaforereferenced US. Pat. No.

The conduction angle over which each phase winding is energized by thecorresponding phase of the supply potential is determined by theelectrical angle during eachpotential cycle of each phase of the supplypotential at which each respective trigger signal is produced, theearlier during eachpotential half cycle that the trigger signal isproduced, the greater the conduction angle. The speed of the'motor, ofcourse, increases with an increase of the conduction angle.

Timing capacitor 25 beginsto charge with the appearance of the referencepotential and triggers unijunction transistor 20 to conduction toproduce a master trigger signal across base resistor 23 at theelectrical angle of each cycle of the reference phase of the supplypotential at which the charge upon timing capacitor 25 has reached amagnitude substantially equal to the peak point potential of unijunctiontransistor 20. To change the conduction angle, therefore, the timerequired for the charge on timing capacitor 25 to reach a magnitudesubstantially equal to the peak point potential of unijunctiontransistor 20 may be varied In capacitor charging circuits, thecapacitor will charge substantially exponentially, to substantially fullAccording to Kirchoffs second law, the .sum of the potential drops inany complete or closed circuit must equal the sum of the potential risesin that circuit. In

applying this law, tracing through any single circuit,

whether it is by itself or a part of a network, a potentialdrop existswhen tracing through a resistance with or inthe same direction as thecurrent or through a battery or generator against their potential, frompositive to negative, and a voltage rise exists when tracing through aresistance against or in a direction opposite to the current, or througha battery or a generator in a direction with their potential, negativeto positive.

Considering Zener diode 24 as a potential generator, the sum of thepotential drops across the parallel combination of capacitor 44,resistor 45 and diode 46in series and diode 47, potentiometer 48 andresistor 49 in tial drop thereacross in this loop.

As the circuit components included in the loop just considered and thefrequency and amplitude of the alternating current supply potential and,therefore, the reference potential, remain substantially constant, thevariable parameter necessary to control motor speed may be a directcurrent control potential, which not only is related to and varies inmagnitude with motor speed but which is also selectively variable inmagnitude for varying the speed of the motor, applied across variableresistance 36.

One method for producing a direct current control potential which varieswith motor speed is an electric tachometer driven by the motor shaftwhich produces a direct current output potential of a magnitudeproportional to motor speed. Commercially available tachometers of thistype are small direct current generators which produce a direct currentoutput potential which increases and decreases linearly in magnitudewith increases and decreases of motor speed. As electrical tachometersof this type are well known in the art and form no part of thisinvention, it has been schematically illustrated as a circle in FIG. 1and referenced by the numeral 9. The output direct current potential oftachometer 9 will hereinafter be referred to as the control potential.

The control potential is connected across the impedance element,variable resistance 36, through a circuit which may be traced from thepositive polarity output terminal of tachometer 9, through potentiometer71, diode 72, lead 73, variable resistance 36 and leads 74 and 50 to thenegative polarity terminal of tachometer 9. The magnitude of the controlpotential may be selectively varied by adjusting the movable contact ofpotentiometer 71 to adjust motor speed. With the control potentialapplied across variable resistance 36 and poled as shown in FIG. 1, thisvariable resistance may be considered a potential generator. Tracingthrough the same loop in the same direction and with the potentialapplied across variable resistance 36 poled as shown in FIG. 1, thepotential appearing across variable resistance 36 is a potential dropfor purposes of Kirchoffs second law. As the reference potential acrossZener diode 24 and the value of the circuit com- 8 ponents remainsubstantially constant, the increase in magnitude of the potential dropacross variable resistance 36 introduced by the control potentialapplied thereacross must be accompanied by a corresponding decrease inpotential drop across timing capacitor 25 to satisfy Kirchoffs secondlaw. As thepotential drop across timing capacitor 25 determines themagnitude of thecharge thereon, the initialrate of charge of timingcapacitor 25 and, therefore, the time required for the charge toincrease in magnitude to the peak point potential of unijunctiontransistor 20 is determined by the magnitude of the difference betweenthe reference and control potentials which may be varied by varying themagnitude of the control potential applied across variable resistance36.

. It is apparent, therefore, that upon motor start, the controlpotential is zero and the magnitude of the diffe'rence between thereference and control potentialsv is maximum. Consequently, the fullcharging potential of the reference potential is applied across timingcapacitor 25 to rapidly charge this device to the peak point potentialof unijunction transistor 20. As point occurs substantially atthebeginning of each potential cycle of the reference phase,substantially full potential is applied across the motor.

To establish the difference of magnitude between the reference potentialand the control potential at substantially zero upon motor start and'toincrease the difference in magnitude at a predetermined rate to amaximum as determined by the selected magnitude of the controlpotential, a capacitor is connected across the series combination of theimpedance element, variable resistance 36 and timing capacitor 25through a circuit which may be traced from junction 39, through diode81, resistor 82, capacitor 75 to point of reference or ground potential5.

Upon motor start, normally closed contact 65, which maybe one of thenormally closed contacts of the associated motor starter relay, isoperated to the circuit open condition. Consequently, capacitor 75begins to charge through diode 8 1 and resistor 82. With capacitor 75charging, junction 39 is at substantially ground potential.Consequently, charging capacitor 75 establishes the difference inmagnitude between the reference and control potentials, and consequentlythe magnitude of the charging potential for timing capacitor 25, atsubstantially zero upon motor start. Capacitor 85, however, begins tocharge through variable resistance 86 and resistor 87 until it hasreached a magnitude equal to the peak point potential of unijunctiontransistor 70. Upon the conduction of unijunction transistor 70,capacitor discharges through the emitter-base. junction thereof and baseresistor 23 to produce a trigger potential signal thereacross of apositive polarity upon junction 51 with respect to point of reference orground potential 5. This trigger signal gates silicon controlledrectifier 40 conductive to disenable the master trigger signal producingcircuit for the remainder of this potential cycle, provides a triggersignal across output terminals 53(1) and 54(1) and initiates the actionof the slave trigger signal circuitry in a manner previously described.By properly adjusting variable resistor 86,, the electrical angle atwhich capacitor 85 has become charged to the peak point potential ofunijunction transistor 70 may be established near the end of eachpositive polarity halfcycle of the reference phase of the supplypotential. Consequently, the trigger signal for each potential cycle ofthe supply potential occurs near the end of each positive polarityhalf-cycle to provide a reduced voltage across the motor upon start.

As capacitor 75 continues to charge and the motor begins to run, thepotential upon junction 39 begins to increase in magnitude and themagnitude of the control potential across variable resistance 36increases. Consequently, the magnitude of the difference between thereference and control potentials and, consequently, the chargingpotential for timing capacitor begins to increase in magnitude. Thisincrease in magnitude charges timing capacitorearlier during eachpotential cycle of the. reference phase of the supply potential toproduce the trigger signals earlier during each potential half-cycle ofeach phase of the supply potential to increase the conduction angle and,consequently, the speed of the motor. The difference in magnitudebetween the reference and control potentials and, consequently, thecharging potential for timing capacitor 25 continues to increase at apredetermined rate as determined by the time constant of the chargingcircuit for capacitor 75 to a maximum value as determined by theselected magnitude of the control potential. Consequently, the speed ofthe motor continues to increase until it is operating at a speed asdetermined by the setting of the movable contact of potentiometer 71 and.capacitor '75 is of no further consequence although it continues totake a charge through diode 90 until charged to the full referencepotential. As the capacitor 75 is no longer effective, the speed controlsystem operates in the normal manner.

Upon the shutdown of the motor, the normally closed contacts 65 of, theassociated controller relay close to discharge capacitor 75 and preparethe system for the next motor start. I

While apreferred embodiment of the present invention has been shown anddescribed, it will be obvious to those skilled in the art that variousmodifications and substitutions may be made without departing from thespirit of the invention which is to be limited only within the scope ofthe appended claims.

What is claimed is:

l. A speed control system for alternating current motors of the typehaving discrete phase windings corresponding to each phase of acompatible alternating current supply potential comprising, means forproducing a reference potential in synchronism with a selected phase ofsaid alternating current supply potential, means for producing a controlpotential which varies in magnitude with changes of motor speed, meansfor selectively varying the magnitude of said control potential forvarying the speed of said motor, means including a unijunctiontransistor type relaxation oscillator circuit having a timing capacitorresponsive to a difference in magnitude between said reference potentialand said control potential for producing a trigger signal at theelectrical angle of each potential cycle of each phase of saidalternating current supply potential as determined by the magnitude ofsaid difference, means for establishing said difference of magnitudebetween said reference potential and said control potential atsubstantially zero upon motor start and for increasing said differencein magnitude at a predetermined rate to 4 a maximum as determined by theselected magnitude of said control potential, and controllable switchingcircuit means corresponding to each phase of said alternating currentsupply potential for completing an energizing circuit for thecorresponding said phase windings in response to respective said triggersignals.

3 2. A speed control system for alternating current motors of the typehaving discrete phase windings corresponding to each phase of acompatible alternating current supply potential comprising, means forproducing a reference potential in synchronism with a selected phase. ofsaid alternating current supply potential, means forproducing a controlpotential which varies in magnitude with changes of motor speed, meansfor selectively varying the magnitude of said controlpotential forvarying the speed of said motor, means including aunijunction transistortype relaxation oscillator circuit having a timing capacitor charged bya charging potential of a magnitude determined by the difference inmagnitude between said reference potential and said control potential.for producing a trigger signal at the electrical angle of eachpotential cycle of each phase of said alternating current supplypotential as determined by the magnitude of said difference, means forincreasing the magnitude of said capacitor charging potential at apredetermined rate from substantially zero upon motor start to a maximumas determined by the selected magnitude of said control potential, andcontrollable switching circuit means corresponding to each phase of saidalternating current supply potential for completing an energizingcircuit for the corresponding said phase windings in response torespective said trigger signals.

3. A speed control system for alternating current motors of the typehaving discrete phase windings corresponding to each phase of acompatible altemating' current supply potential comprising, means forproducing a reference potential'in synchronism with a selected phase ofsaid alternating current supply potential, means for producing a controlpotential which varies in magnitude with changes of motor speed, meansfor selectively varying the magnitude of said control potential forvarying the speed of said motor, means including a unijunctiontransistor type relaxation oscillator circuit having a timing capacitorresponsive to a difference in magnitude between said reference potentialand said control potential for producing-a trigger signal at theelectrical angle of each potential cycle of each phase of saidalternating current supply potential as determined by the magnitude ofsaid difference, an impedance element, means for connecting saidreference potential across the series combination of said impedanceelement and said timing capacitor, means for connecting said controlpotential across said impedance elementin the same polarity relationshipas said reference potential, a capacitor, means for connecting saidcapacitor across said series combination of said impedance element andsaid timing capacitor, and controllable switching circuit meanscorresponding to each phase of said alternating current supply potentialfor completing an energizing circuit for the corresponding said phasewindings in response to respective said trigger signals.

1. A speed control system for alternating current motors of the typehaving discrete phase windings corresponding to each phase of acompatible alternating current supply potential comprising, means forproducing a reference potential in synchronism with a selected phase ofsaid alternating current supply potential, means for producing a controlpotential which varies in magnitude with changes of motor speed, meansfor selectively varying the magnitude of said control potential forvarying the speed of said motor, means including a unijunctiontraNsistor type relaxation oscillator circuit having a timing capacitorresponsive to a difference in magnitude between said reference potentialand said control potential for producing a trigger signal at theelectrical angle of each potential cycle of each phase of saidalternating current supply potential as determined by the magnitude ofsaid difference, means for establishing said difference of magnitudebetween said reference potential and said control potential atsubstantially zero upon motor start and for increasing said differencein magnitude at a predetermined rate to a maximum as determined by theselected magnitude of said control potential, and controllable switchingcircuit means corresponding to each phase of said alternating currentsupply potential for completing an energizing circuit for thecorresponding said phase windings in response to respective said triggersignals.
 2. A speed control system for alternating current motors of thetype having discrete phase windings corresponding to each phase of acompatible alternating current supply potential comprising, means forproducing a reference potential in synchronism with a selected phase ofsaid alternating current supply potential, means for producing a controlpotential which varies in magnitude with changes of motor speed, meansfor selectively varying the magnitude of said control potential forvarying the speed of said motor, means including a unijunctiontransistor type relaxation oscillator circuit having a timing capacitorcharged by a charging potential of a magnitude determined by thedifference in magnitude between said reference potential and saidcontrol potential for producing a trigger signal at the electrical angleof each potential cycle of each phase of said alternating current supplypotential as determined by the magnitude of said difference, means forincreasing the magnitude of said capacitor charging potential at apredetermined rate from substantially zero upon motor start to a maximumas determined by the selected magnitude of said control potential, andcontrollable switching circuit means corresponding to each phase of saidalternating current supply potential for completing an energizingcircuit for the corresponding said phase windings in response torespective said trigger signals.
 3. A speed control system foralternating current motors of the type having discrete phase windingscorresponding to each phase of a compatible alternating current supplypotential comprising, means for producing a reference potential insynchronism with a selected phase of said alternating current supplypotential, means for producing a control potential which varies inmagnitude with changes of motor speed, means for selectively varying themagnitude of said control potential for varying the speed of said motor,means including a unijunction transistor type relaxation oscillatorcircuit having a timing capacitor responsive to a difference inmagnitude between said reference potential and said control potentialfor producing a trigger signal at the electrical angle of each potentialcycle of each phase of said alternating current supply potential asdetermined by the magnitude of said difference, an impedance element,means for connecting said reference potential across the seriescombination of said impedance element and said timing capacitor, meansfor connecting said control potential across said impedance element inthe same polarity relationship as said reference potential, a capacitor,means for connecting said capacitor across said series combination ofsaid impedance element and said timing capacitor, and controllableswitching circuit means corresponding to each phase of said alternatingcurrent supply potential for completing an energizing circuit for thecorresponding said phase windings in response to respective said triggersignals.